[Backport] Set fallback values for email and _website columns to avoid 'undefined index' error in CustomerComposite.php

[TomashKhamlai]

Original Pull Request

#18679

Description

There are proper validation messages for ERROR_CODE_COLUMN_NOT_FOUND and ERROR_CODE_COLUMN_NAME_INVALID but this code is not reachable.

Manual testing scenarios

1. Create a Customer
2. Navigate to Customers -> All Customers and export them all
3. Navigate to System -> Import and use the CSV of all your customers. Use Add/Update behavior and "Customers and Addresses (single file)" as an entity type.

Expected result:
At least one of the two validation messages:

We can't find required columns: email", "_website.

Column names: "ID", "Name", "Email", "Group", "Phone", "ZIP", "Country", "State/Province", "Customer Since", "Web Site", "Confirmed email", "Account Created in", "Billing Address", "Shipping Address", "Date of Birth", "Tax VAT Number", "Gender", "Street Address", "City", "Fax", "VAT Number", "Company", "Billing Firstname", "Billing Lastname", "Account Lock" are invalid

Actual result:
Notice: Undefined index: email in /var/www/html/pr/app/code/Magento/CustomerImportExport/Model/Import/CustomerComposite.php on line 302

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @TomashKhamlai. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

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[rogyar]

@magento-engcom-team give me 2.3-develop instance

[magento-engcom-team]

Hi @rogyar. Thank you for your request. I'm working on Magento 2.3-develop instance for you

[magento-engcom-team]

Hi @rogyar, here is your Magento instance.
Admin access: http://ec2-34-228-235-121.compute-1.amazonaws.com/i-18681-2-3-develop//admin
Login: admin Password: 123123q
Instance will be terminated in up to 3 hours.

[magento-engcom-team]

Hi @TomashKhamlai. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.